AC conductivity evolution in bulk and grain boundary response of sodium tungstate Na2WO4

The Na2WO4 compound has been obtained by the conventional solid-state reaction and characterized by X -ay powder diffraction. The title material crystallizes in the cubic system with Fd-3m space group. The electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz-5 MHz and temperature range 586-679 K. Two semicircles are observed in impedance plot indicating the presence of two relaxation processes in the compound associated with the grain and grain boundary. The relaxation behavior of the grain and grain boundary of the Na2WO4 are also obtained from the analyzed electrical modulus data. AC conductivity measured follows the power-law dependence sigma(AC)similar to omega(s) typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process. The mechanism of conduction is probably due from the displacements of the Na+ ion in the tunnel-type cavities along [111] direction.